In recent years, there is a demand to transmit large capacity data at high speed in data transmission in the device. To respond to such demand on high speed transmission of the large capacity data, light transmission that enables high speed transmission of data is used. Such light transmission has an advantage in that the signal of the data to be transmitted can be transmitted through the light transmission path (light guide) at high speed and without degrading.
If high speed transmission of data is not demanded, transmission by electrical signal as in the related art is preferably used as the power consumed for the signal transmission is smaller than the light transmission.
A so-called electrical/light complex transmission system in which the light transmission and the transmission by electrical signal can be simultaneously used has been proposed (e.g., patent documents 1 and 2).
In the system shown in patent document 1, electronic control units 101, 102 including an integrated circuit such as a CPU 111, 121 are coupled by optical fibers 103, 104 serving as light transmission paths and a start signal line 105 serving as an electrical transmission path, as shown in FIG. 36.
In the above-described electrical/light complex transmission system, a plurality of transmission paths is normally used in the data transmission in the device as in the portable telephone device of patent document 2 shown in FIG. 37. Such portable telephone device has a plurality of transmission paths coupled with an integrated circuit (control unit 205) for controlling input/output of electrical signals in each transmission path.
In order to reduce the circuit configuration of the electrical/light complex light transmission system, the input electrical signal is preferably switchable so as to be light-transmitted according to the characteristics such as the frequency of the electrical signal. That is, the circuit configuration of the electrical/light complex light transmission system can be reduced by being configured such that the light transmission path for transmitting and receiving electrical signals is commonly used between a plurality of electrical signal input units and a plurality of electrical signal output units, and only the electrical signal that is desirably transmitted at high speed is light-transmitted.
The configuration of such electrical/light complex light system may be a branched configuration in which the electrical signal of high frequency of the electrical signals input from the input terminal 301 . . . is transmitted in the light transmission path 304, and the electrical signal of low frequency is transmitted in the electrical transmission path 305, as shown in FIG. 38.
The electrical/light complex system includes a high pass filter (HPF) 302 for passing only the signal of high frequency and a low pass filter (LPF) 303 for passing only the signal of low frequency. In a case where the electrical signal input from an input terminal 301 is a signal of high frequency, the electrical signal can pass through the high pass filter 302, but cannot pass through the low pass filter 303. Thus, the electrical signal of high frequency is transmitted towards an output terminal 308 via a circuit line including an E/O (Electrical/Optical) converter 306 and an O/E (Optical/Electrical) converter 307 and arranged with the light transmission path 304.
Therefore, only the electrical signal of high frequency of the electrical signals input from the input terminal 301 can be light-transmitted by including the high pass filter 302 and the low pass filter 303.
However, in the conventional configuration capable of transmitting only the electrical signal of high frequency by the light transmission path using the high pass filter and the low pass filter, the entire circuit configuration related to such data transmission cannot be miniaturized and the power consumption cannot be reduced.
That is, the circuit configuration of the high pass filter includes a capacitor (C) on the electrical transmission path and sandwiches the electrical transmission path with resistors (R) so that voltage is applied on the path, as shown in FIG. 34. The circuit configuration of the low pass filter includes a capacitor (C) between the electrical transmission path and a GND transmission path, as shown in FIG. 35.
Therefore, the high pass filter has a larger circuit configuration than the low pass filter. Furthermore, the high pass filter also has a problem in that the power consumption increases compared to the low pass filter since voltage needs to be applied to the path to define the potential of a center value of the high frequency signal.
Therefore, the circuit configuration of the high pass filter is large and the power consumption increases even with the configuration of commonly using the light transmission path among a plurality of electrical signal input units and a plurality of electrical signal output units to reduce the circuit configuration related to data transmission, whereby sufficient miniaturization cannot be achieved and the power consumption cannot be reduced.    Patent document 1: Japanese Unexamined Patent Publication No. 5-193427 (date of publication: Aug. 3, 1993).    Patent document 2: Japanese Unexamined Patent Publication No. 2003-244295 (date of publication: Aug. 29, 2003).